There exists today a need to provide semiconductor packages which have a very great number of leads but which are also of a very small overall size. Packages manufactured using the well known ceramic multilayer co-fired technology are presently capable of the greatest lead density. Furthermore, ceramic multilayer co-fired packages can also provide the additional benefit of having ground and power planes and the attendant advantages thereof. However, ceramic multilayer co-fired packages are prohibitively expensive except for applications which can command an extremely high price, such as in the construction of devices which must meet military specifications.
Lead count density of less expensive packages, such as plastic or cerdip packages, is limited by the number of lead fingers which can be attached at the die cavity (the smallest area in the package at which leads must be attached). Co-fired packages also face similar limitations as to the quantity of leads which can be attached at a die cavity, but this limitation is overcome by the fact that the co-fired technology allows for providing more than one tier of bonding locations at the die cavity perimeter.
Recent advances in techniques for etching or stamping lead frames has allowed lead frame tips of less than 0.015 cm (0.006 in) to be produced thus providing, at the extreme, lead counts even exceeding 400 with a conventional die cavity size and configuration. However, these advances alone will still not always provide the lead densities required in many present and future applications. Furthermore, lead frames having such high lead counts require very expensive dies if they are to be produced by stamping. Indeed, the tooling cost for such lead frames is often so expensive that it is only practical to produce them by the etching process, even though the per piece cost is much greater than can be achieved using stamping methods.
Another factor affecting the cost of semiconductor packaging is the fact that lead frames must often be treated with a non-oxidizing substance, such as gold, or with a minimally oxidizing metal, such as aluminum, at their die cavity ends to allow attachment of connection interconnecting wires by conventional interconnection methods, such as aluminum ultrasonic bonding, gold thermo-sonic bonding or TAB (tape automated bonding). For DIP (dual in line packages), this can be accomplished by stamping the lead frames from a metal strip or coil having an aluminized stripe running at its center. However, for quad packs this method cannot be used, since the aluminized stripe on the web would necessarily run through two of the four sides of any lead frame stamped from such material. Today, four sided lead frames are stamped from non-aluminized material, and then aluminum is vapor deposited on only the lead tip areas, at a considerably greater cost.
Obviously, it would be desirable to produce a package which could incorporate the advantages of a very high lead count and the possibility of inclusion of voltage and ground planes at a cost substantially below that of co-fired packages. Even more benefit could be derived if such packages could utilize lead frames which could be stamped using less expensive dies than those normally used, and which could be stamped from material on which an aluminum layer was coextruded or vapor deposited in a high volume manufacturing process, thus eliminating a need for the expensive process of vapor aluminizing the lead frame bond tips after they are stamped. Alternatively, if it is decided to produce the lead frames by etching, it would be advantageous if more frames could be produced per sheet, since the bulk of costs associated with etching are accrued on a per sheet basis.
All of the prior art semiconductor packages within the inventors' knowledge have required extremely expensive packaging styles to reliably produce very high lead densities which are capable of meeting the necessary stringent electrical parametric requirements, such as low inductance of conductor traces, low resistance of conductor traces, and appropriate shielding of voltage lines.
No prior art semiconductor package to the inventors' knowledge has successfully combined very high lead densities with reliability and economy of manufacture. All successful applications to date have either utilized the very expensive ceramic co-fired technology to achieve high lead count density, or else have not provided ground and power planes and have been capable of having the lead density available with ceramic co-fired packages.